Rollup merge of #79149 - sexxi-goose:upvar_ref, r=nikomatsakis

Rollup merge of #79149 - sexxi-goose:upvar_ref, r=nikomatsakis

Move capture lowering from THIR to MIR

This allows us to:
- Handle precise Places captured by a closure directly in MIR. Handling
  captures in MIR is easier since we can rely on/ tweak PlaceBuilder to
  generate `mir::Place`s that resemble how we store captures (`hir::Place`).

- Handle `let _ = x` case when feature `capture_disjoint_fields`
  is enabled directly in MIR. This is required to be done in MIR since
  patterns are desugared in MIR.

Closes: rust-lang/project-rfc-2229#25
r? ```@nikomatsakis```